To switch to the preview mode, where you can see the UI in a device simulator, click on the switch button in the top right to enable Test, as illustrated:

In this mode, you should be able to interact with the components in the web browser as if they're actually deployed on the device.

If you break something, it's very simple to start a new project. It's a great tool to use for prototyping and to get the initial template or project scaffolding. You should continue coding in your regular IDE for the rest of the app. Ionic Creator doesn't do everything for you yet. For example, if you want to access specific Cordova plugin features, you have to write that code separately.

Also, if you want to tweak the interface outside of what is allowed within Ionic Creator, it will also require specific modifications to the .html and .css files.